The present invention relates generally to battery chargers and more specifically to a method of preventing damage to batteries during charging.
In the field of battery chargers for satellite applications, a charger is typically employed to transfer electrical energy generated by solar panels to chemical energy stored by battery packs. Many satellites experience an eclipse season where relative motion of the body that it orbits, such as the earth, results in the body being between the satellite and the sun. Typical eclipse duration for a satellite in geo-synchronous earth orbit is 1.2 hours, or about 1 hour, 12 minutes. During an eclipse, the batteries may power the operation of required experiments, correctional navigation, communications, or other vital functions. In order to power the operations of the satellite, it is generally desired that the batteries be fully charged at the beginning of each eclipse. A battery that is not adequately charged at the outset of an eclipse may result in unwanted conditions that necessitate limited operations or a total loss of the satellite.
A concern for battery charger control is preventing overcharge. Briefly, when a battery is fully charged, the cells have chemically stored a desired amount of electrical energy. As more energy is transferred to the cells through overcharging, undesirable chemical reactions within the cells typically result in a temperature rise. Severe battery overcharge can produce high battery temperatures, which may lead to an inability to charge the battery. In a satellite application, severe overcharge can also result in operational difficulties with the batteries for several days. Severe battery overcharge can also result in battery damage, fire, disruption of service, or catastrophic loss of a satellite.
Conventional battery chargers for satellite applications incorporate a thermal switch into each battery to prevent overcharge. The thermal switch is typically preset to terminate the charging cycle at a preselected battery temperature. This method takes advantage of the fact that battery cell temperature will increase during overcharge. One drawback to the use of a thermal switch is that it does not compensate for the temperature of the battery at the outset of the charging cycle. If the battery is at a relatively low temperature at the beginning of a charge cycle, the battery may be allowed to severely overcharge as battery temperature increases to the thermal switch set point. If the battery temperature is high at the beginning of the charge cycle, either due to a discharge or as a result of heat generated by other sources such as the sun, microwave amplifiers or other batteries, all of which can provide excess heat to a satellite, then the thermal switch may terminate the charging cycle prematurely.
Yet another concern for battery chargers in satellite applications is the lack of protective systems in the case of a failure within the charging system. One failure of concern is related to continuous battery charging. A failure that results in continuous battery charging may result in the same undesirable conditions as severe battery overcharge. What is needed, therefore, is a battery overtemperature control that will account for battery temperature rise during charging to prevent overcharging. A favorable battery overtemperature control would also be capable of preventing a battery overcharge in the event of selected battery charger failure modes.